The present invention relates generally to radar systems and more specifically the invention pertains to a system which produces low sidelobe levels in array antennas. The center elements of the array have a uniform current amplitude weighting while the edge elements have a current amplitude taper. Tapering the current of the edge elements produces a low sidelobe far field pattern with a simplified feed network.
Microwave radar and communication antennas emit radio frequency (RF) signals in a beam which, when directed, has a main lobe and sidelobes. The main lobe is the useful portion of the emitted radiation which can track a target for a radar system or any useful information for a communication system.
Sidelobes are undesired radiation phenomena which use up the energy of an antenna, and which are capable of generating interference. Current methods for constructing low sidelobe antennas for radar systems include: amplitude tapering the array; phase tapering the array; nonuniform element spacing; and using a combination of the above techniques. Each of these approaches are discussed below.
Amplitude tapering the array is performed when the current fed to the array elements is tapered from a maximum at the center to a minimum at the edges. Common amplitude tapers include Dolph-Chebychev, Taylor, and binomial. These tapers may be found in most antenna books. These tapers require complicated feed networks that are difficult to build and test.
Phase tapering the array is performed when the phase of the current at each element in the array is tapered. This technique is not commonly used and requires a large array. Only a limited amount of sidelobe control is possible.
Nonuniform element spacing is performed when the elements in the array are either randomly spaced or unequally spaced in a manner that produces low sidelobe levels. These arrays are difficult to build and test. Low sidelobes are difficult to obtain and grating lobes are a problem. A combination of the above techniques is very complex and is seldom used.
The task of reducing sidelobes is alleviated, to some extent, by the systems disclosed in the following U.S. Pat. Nos., the disclosures of which are incorporated herein by reference:
U.S. Pat. No. 3,780,373 issued to Unz; PA0 U.S. Pat. No. 3,553,706 issued to Charlton; PA0 U.S. Pat. No. 2,286,839 issued to Schelkunoff; and PA0 U.S. Pat. No. 1,643,323 issued to Stone
The patents identified above relate to the use of amplitude control in antenna arrays to achieve certain performance characteristics. In particular, the Unz patent describes a nonuniformly optimally spaced antenna array where the optimum positions were systematically determined by a synthesis technique. Both the optimum position for each array antenna and the corresponding amplitude are determined by solving a linear equation involving complex Fourier series coefficients.
The Charlton patent relates to an antenna array where the radiating elements are arranged into group of elements. In each group, the elements are arranged to be symmetrical about three axes having a common intersection and commonly excited, and each group of elements different basic is excited by a different basic excitation signal.
The Schelknunoff patent describes a method and means for obtaining optimum prescribed directive action from a linear antenna array. Both the spacing of array elements and the amplitude distribution are determined by the complex coefficients of a polynominal expansion, which is presented in the patent.
The Stone patent relates to a method and means for a directive radio transmitting and receiving station. A directive antenna array is disclosed which radiates and receives only in a desired direction. To accomplish this uni-directional transmission or reception, the intensities of excitation along the rows and columns of the array are graded according to the coefficients of a polynomial expansion, which is presented in the patent.
While the above-cited references are instructive, a need remains to use amplitude tapering of phased array antennas to produce low sidelobe levels. The present invention is intended to satisfy that need.